UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker had his moments, delivering effective clean blows in a fight where the approaches failed to mesh, with constant grabbing marring the fight.
The scoring judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the ring as he was met by boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Several boxing experts believed the challenger, thirty-one, deserved the victory but the contender suffered the second loss of his 28-bout career.
The Champion Earns Victory in Slow Encounter
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to edge the better periods.
The audience's tolerance diminished amid frequent holding, with Parker going down various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight title.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells tolled in memory of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime faded, the quiet became a shout of "there's just one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was knocked down by a powerful punch directly to the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was marked up early, heavy swelling appearing under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor barely secured the result with official scores of draw, narrow lead and clearer margin, but each competitor were relentless and full of heart, everything their hero Hatton stood for.
